Strong Letter or Experience

[IamWitten]

I have two options for a summer internship. One is a reading project with a professor at my university and who has a good impression of me, even though I didn't take any of his courses. I am very confident that he'll write good LOR. Other option is an internship at one of the top research institutes in my country, with a professor I don't know. I did some research and found that the second prof is very chilled and also might write a good recommendation. But the subject in which I am doing an internship (at the top institute) is not really my strongest but I'll have an amazing thing to put on my CV, if I did. 

 

What should I do ?